The invention relates generally to steam turbines and more specifically to a method for operating steam turbines with transient elevated back pressure.
Condensing steam turbines are provided with allowable exhaust pressure operation guidelines which are intended to protect the steam turbine from potential damage that could occur with excessively high exhaust (condenser) pressure. The guidelines include exhaust pressure alarm and trip limits that are usually presented as a function of the last stage bucket's (LSB) annulus velocity (VAN). Annulus velocity is the steam velocity measured at the exit of the bucket and is a direct function of the flow through the last stage, its area, and the inverse of condenser pressure.
Turbine stage pressure varies approximately linearly with flow. For representation of steam velocity (flow), stage pressure may be monitored at various points for use by the turbine control system as an indication of the last stage bucket flow. Typically, reheat or intermediate pressure (IP) turbine exhaust or low-pressure (LP) turbine inlet pressure is used to determine flow in the LP turbine. Steam turbines with low-pressure extractions may use the LP extraction pressure as the indicator of last stage flow. If the actual turbine exhaust pressure exceeds the alarm setting for a given calculated annulus velocity, a turbine control system may provide the operator a warning that the actual exhaust pressure exceeds the limit so that the operator can take alternative action if possible. The turbine control system may permit continuous operation of the unit in the alarm region of allowable exhaust pressure operation curve without tripping the unit. However if the measured exhaust pressure, exceeds the trip setting for a given calculated annulus velocity, the turbine control system will immediately trip the unit by closing the steam control valves. These valves providing quick closing type operation may be called various names, such as admission valves, cutout valves, control valves, trip valves, turbine trip and control valves, etc.
In establishing allowable exhaust pressure operating guidelines, it has been recognized that the potential of last stage bucket damage increases at lower annulus velocities and is highest at combinations of low flow and higher exhaust pressure. Consequently variable operational limits apply with lower allowable pressures at very low VAN, and higher allowable exhaust pressures at higher VAN with a varying pressure in between. Existing alarm and trip curves for steam turbine operation for exhaust pressure limits versus VAN incorporate the more restrictive exhaust pressure requirements at lower values of VAN.
During a load rejection, steam turbine flow is rapidly reduced by the main steam turbine control valves and control valves for reheat and LP admission (if applicable) to prevent turbine overspeed. The turbine load is reduced to no load or house load and because these loads are very small, the VAN is almost zero due to the very low flow.
For units with water-cooled condenser, and units without turbine bypass systems, the exhaust pressure transients associated with load rejections has generally not been an issue, because exhaust pressure transients have remained within the allowed exhaust pressure limits. However, for units with air cooled condensers and/or bypass systems the resulting pressure transient is more problematic and the potential for a trip during the load rejection due to high exhaust pressure is increased. For units with variable exhaust pressure operation limits this has been a larger issue due to lower allowed exhaust pressures at low VAN compared to the allowed exhaust allowed at full load.
Even though turbine steam flow is reduced, bypass systems divert boiler steam to the condenser increasing condenser pressures. This diverted flow results in increased heat rejection to the condenser and higher exhaust pressures. Runback of the heat source (e.g. supplemental firing or gas turbine runback for combined cycle units, or firing for fossil units) may result in an exhaust pressure spike. FIG. 1 illustrates a simulation exhaust pressure and VAN following a load rejection. At t=0, VAN 10 drops close to 0 and exhaust pressure spikes 20. Over several minutes the exhaust pressure transient decays 30. If the pressure exceeds a limit (not shown), the current steam turbine control system logic will trip the unit even though this spike may be of short duration.
One required element for control of steam turbines is to avoid an overspeed trip under the condition of a full load rejection. This requirement is prescribed in IEEE-122, “IEEE Recommended Practice for Functional Performance characteristics of Control Systems for Steam Turbine Generators”, or similar regulatory documents. Contractually, this requirement is often expanded to specify that the steam turbine be capable of a full load rejection without initiating a turbine trip.
Accordingly, it would be desirable to maintain the steam turbine in a safe condition and at the same time avoid a turbine exhaust vacuum trip due to a short duration condenser pressure transient caused by a large reduction in turbine flow coupled with a transient increase in exhaust pressure, which specifically occurs during full load rejections.